<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    *{
      margin:0;
      padding:0;
    }
    html {
      font-size: calc(100 / 750 * 100vw);
      /* font-size: 13.33333333vw; */
    }

    .box {
      width: 1rem;
      height: 1rem;
      background: red;
    }

  </style>
</head>
<body>
<div class="box"></div>
</body>
</html>
